You may want to consider getting a little rock paper scratch pad like the velle book or rocketbook. Or whatever cheap alternative you find. The paper is waterproof and if you use frixion pens, can be wet erased or erased with the special nub on the pen. I just run the whole thing under the faucet when it’s full and wipe it down. Or if I’m feeling fidgety, I’ll clean each page individually. Also great for doodling.

Something tiny to keep in a pocket, might work well for your needs since the note itself isn’t important. It works decently for mine.